The present invention generally relates to an electro-hydraulic braking system in which the braking force acting upon a service brake is generated by an electrically controlled pump.
A simplified version of this system is shown in FIG. 1. This system 10 comprises a pump 12 powered by a motor 14 in response to control signals by an electric control unit (ECU) 16. The pump 12 directly pressurizes a brake system generally shown as 18. The speed of the motor 14/pump 12 is controlled to modulate brake system pressure in accordance with a commanded brake pressure signal. In the system shown in FIG. 1, brake system pressure varies with motor speed. In order for the brake system pressure to decay or decrease quickly, fluid within the brake system must flow to a drain or sump 22. The speed of the pressure decay can of course be increased dramatically if the pump direction is reversed, however, this adds to the complexity of the system. As can be appreciated, when the pump speed is slowed or when the pump is brought to a halt, brake system pressure will decay through the internal leak passages 24 of the pump, however, the time required for such decay is relatively long. One solution is to add an orifice (an additional leak path) in parallel with the pump 12 to speed the brake pressure decay. This approach requires that the pump capacity or displacement be increased since the pump now must be sized to supply the leakage flow as well as to supply fluid to adequately pressurize the brake system 18. If a fixed orifice 26 is used, it is impossible to size this orifice to provide a sufficient decay at low brake system pressure without increasing the flow at high pressure to a point that the pump 12 cannot maintain this pressure. One approach to solving the problem at hand is to add a flow regulator in a passage such as the passage in which the orifice 26 is located. One problem with this approach is that during the brake apply portion of the braking cycle the pump 12 is required to provide flow to the brake as well as to supply bypass flow.
It is an object of the present invention to provide an improved direct acting electro-hydraulic braking system. In this improved system a flow regulator is added to the braking system shown in FIG. 1 to modulate the leak rate. The time required to decay brake pressure at low pressure levels is reduced and at the same time, leakage at high brake system pressures is minimized. The present invention provides a simple and compact way to regulate a leak or bypass rate at all operational pressures. In the preferred embodiment of the present invention a flow regulator is added to the system to modulate the leak rate, the decay at low pressure is improved and at the same time leakage at high pressures is minimized. Therefore, when the pump output flow is very high (during the fill mode) the bypass flow is completely shut off. After brake pressure reaches the desired value and the pump slows, the regulator starts to increase the bypass rate. If the pump is stopped completely (when a lower brake pressure is desired) the bypass is open fully, causing a rapid reduction in brake pressure. Alternate embodiment of the invention shows an inline flow regulator.
As will be shown from the description below, brake pressure decay can be achieved without the use of auxiliary control valves and by modulating motor speed is all that is required to obtain a full range of apply, hold and decay rates in brake pressure.
Accordingly, the invention comprises: a system and method of controlling pressure in a wheel or brake cylinder of a brake. In the preferred embodiment of the invention a hydraulic wheel cylinder is connected to an electrically driven pump and a flow control valve is connected between the pump and the wheel cylinder. In one embodiment of the invention the flow control valve comprises a flow control regulator with a bypass passage. In another embodiment of the invention an inline flow control regulator is used. The system and method operate to control the speed of a motor driven pump to generate a desired pressure in a cylinder of the brake. The controlled pump flow generates a differential pressure in a flow control valve as a function of pump flow. This pressure differential is communicated to a spring biased valving element, which is part of the flow control valve, that causes the valving element to move, in response to pressure forces acting on same, generated by the pressure differential, and the force of the bias spring. The valve element, in response to these forces selectively opens and closes an associated exhaust port, connected to a drain or sump, such that in the steady state an equilibrium condition is achieved where the pressure forces balance the spring force and the pressure in the cylinder is constant.
Many other objects and purposes of the invention will be clear from the following detailed description of the drawings.